See Data Port Connection Issues

There are some things related to serial data ports, in particular on Flatpak.

1. There is a bug: https://github.com/OpenCPN/OpenCPN/issues/2528-
2. The port must be accessible. See https://github.com/OpenCPN/OpenCPN/b...NUX_DEVICES.md, the chapter on Device Access.
3. Even if the port is accessible it might not be visible in the UI. Two solutions.
4. The first is to write the name of the device, perhaps something like /dev/ttyUSB0 in the GUI (one don't have to select from the drop-down list, it's possible to enter the device name as well)
5. The second is to create a device link as described in https://github.com/OpenCPN/OpenCPN/b...NUX_DEVICES.md, the chapter on Fixed Device Names. Such links are visible in the GUI